1. Field of the Invention
The present invention relates to high-frequency couplers and, in particular, to high-frequency couplers and communication devices capable of being used in communication of large volumes of data over short distances.
2. Description of the Related Art
In recent years, communication systems in which broadband frequencies are used to transfer large volumes of data, such as images or music, by transmission and reception of radio signals have been attracting attention. By using such a communication system, a large volume of data on the order of 500 Mbps can be transmitted and received over a short distance (on the order of 30 mm) by using a broad frequency band of 1 GHz and higher.
Generally, when an electric field coupling system or an electromagnetic induction system is used for couplers (antennas) for performing communication using high-frequency signals, the energy decreases in proportion to the communication distance. It is known that the energy decreases in proportion to the cube of the distance in electric field coupling. In contrast, the energy decreases in proportion to the square of the distance in magnetic field coupling. This makes it possible to perform communication over a short distance without receiving interference from other communication devices. When communication is performed using high-frequency signals of 1 GHz or higher, since the wavelength of high-frequency signals is relatively short, transmission loss is generated in accordance with the distance. Consequently, there is a need to transmit high-frequency signals efficiently.
As described in Japanese Unexamined Patent Application Publication No. 2008-99236, a high-frequency coupler, in order to communicate a large volume of data between information appliances using a communication system in which broadband frequencies are used, transmits energy primarily through electric field coupling. However, the energy decreases in proportion to the cube of the distance in electric field coupling and, therefore, since the communication distance is also considerably decreased when the size of couplers is reduced, it has been difficult to reduce the size of couplers. Furthermore, a parallel inductor is provided in the high-frequency coupler described in Japanese Unexamined Patent Application Publication No. 2008-99236 in order to improve the transmission efficiency. However, there have been problems in that a certain thickness is required in order to provide a parallel inductor and, moreover, it is also necessary to provide a ground electrode to connect the parallel inductor to the ground, which results in the size of the coupler itself being increased.